The word fatigue is used in everyday living to
describe a range of afflictions, varying from a
general state of lethargy to a specific
work-induced burning sensation within one's
muscles. Physiologically, "fatigue" describes
the inability to continue functioning at the
level of one's normal abilities due to an
increased perception of effort. Fatigue is
ubiquitous in everyday life, but usually becomes
particularly noticeable during heavy exercise.

Two Main Types Of
Fatigue

Central Fatigue The central component to
fatigue is generally described in terms of a
reduction in the neural drive or nerve-based
motor command to working muscles that
results in a decline in the force output. It
has been suggested that the reduced neural
drive during exercise may be a protective
mechanism to prevent organ failure if the
work was continued at the same intensity.
The exact mechanisms of central fatigue are
unknown although there has been a great deal
of interest in the role of serotonergic
pathways.

Peripheral Fatigue Fatigue during
physical work is considered an inability for
the body to supply sufficient energy to the
contracting muscles to meet the increased
energy demand. This is the most common case
of physical fatigue--affecting a national
average of 72% of adults in the work force
in 2002. This causes contractile dysfunction
that is manifested in the eventual reduction
or lack of ability of a single muscle or
local group of muscles to do work. The
insufficiency of energy, i.e. sub-optimal
aerobic metabolism, generally results in the
accumulation of lactic acid and other acidic
anaerobic metabolic by-products in the
muscle, causing the stereotypical burning
sensation of local muscle fatigue.